The most elegant explanation in social psychology convinced me to pursue a Ph.D. in the field. Every few years, a prominent tragedy attracts plenty of media attention because no one does anything to help. Just before sunrise on an April morning in 2010, a man lay dying on a sidewalk in Queens. The man, a homeless Guatemalan named Hugo Alfredo Tale-Yax, had intervened to help a woman whose male companion began shouting and shaking her violently. When Tale-Yax intervened, the man stabbed him several times in the torso. For ninety minutes, Tale-Yax lay in a growing pool of his own blood as dozens of passers-by ignored him or stared briefly before continuing on their way. By the time firefighters arrived to help, the sun had risen and Tale-Yax had died.

Almost half a century earlier another New Yorker, Kitty Genovese, was attacked and ultimately killed while dozens of onlookers apparently failed to intervene. A New York Times writer decried the callousness of New Yorkers and experts claimed that life in the city had rendered them soulless. Just as commentators said in response to Tale-Yax's death, experts wondered how dozens of people with functioning moral compasses could possibly fail to help someone on the verge of death.

Social psychologists are taught to overcome the natural tendency to blame people for apparently bad behavior, and to look for explanations in the environment instead. Witnessing the vocal response to Genovese's death, social psychologists John Darley and Bibb Latane were convinced that something else about the situation explained why the bystanders failed to intervene. Their elegant insight was that human responses aren't additive in the same way that objects are additive. Whereas four light bulbs illuminate a room more effectively than three light bulbs, and three loudspeakers fill a room with noise more effectively than two loudspeakers, two people aren't always more effective than a single person. People second-guess situations, they stop to make sense of a chain of events before acting, and sometimes pride and the fear of looking foolish prevent them from acting at all.

In a series of brilliant studies, Darley and Latane videotaped students as they sat in a room that slowly filled with smoke. The experimenters pumped smoke into the room with a smoke machine, hidden behind a door in a room nearby, but the effect suggested that the door may have concealed a fire. When the students sat in the room alone, they usually left the room quickly and told the experimenter that something was amiss; but when the students sat in small groups of two, three or four, they often remained seated even as they lost sight of the other students through the pall of smoke. When interviewed later, the students said they chose not to act because they were embarrassed, because they weren't sure whether the smoke truly signaled an emergency, and because they relied on the other impassive students in the room to help them decide that the smoke was benign.

According to Darley and Latane, the patterns of thinking that distinguish us from objects and lower-order animals ultimately undermine our willingness to help when we try to understand those situations alongside other people who are equally confused.